Incremental position measuring devices are used in the field of automated manufacturing technology, and in particular in machine tools, for the purpose of measuring changes in the positions of movable parts. For example, incremental rotary encoders measure rotatory movements of rotating shafts. Incremental linear encoders, on the other hand, measure linear displacements of machine components that are situated so as to allow these components to move relative to one another.
In conventional incremental position measuring devices, a detector unit scans a scale division track that includes evenly placed code elements. A wide variety of physical scanning principles may be used for this purpose, such as optical, magnetic, inductive, or capacitive principles. Preferably, the detector signals resulting from the scanning are largely sinusoidal at a uniform movement (constant speed or constant rate of rotation); the positional information is able to be obtained by counting the cycled-through signal periods, for example, or if greater resolution is demanded, also by subdividing the signal periods into a number of linear or angle segments (interpolation). An item of directional information may be obtained if two detector signals that feature a phase shift from each other, e.g., a 90° phase shift, are generated during the scanning. Often, a reference pulse is generated at at least one position in order to produce an absolute reference point for the—design-related—relative—relative position measurement of incremental position measuring devices. A suitable graduation structure, which is likewise scanned by the detector unit, may be arranged on a separate scale division track for this purpose.
The detector signals obtained by the detector unit are processed in a signal processing unit and adapted in accordance with a specification of an output interface. For example, certain interfaces for incremental position measuring devices demand a peak-to-peak value of 1V for the incremental signals. The signals are largely sinusoidal at a constant movement speed (rate of rotation) and have a symmetrical characteristic around a reference potential (frequently the ground potential 0V). The phase shift between the incremental positional signals is 90°. Reference pulse R1 is symmetrical, and its maximum is at a defined position in relation to the incremental positional signals.
The signal processing unit is able to compensate for age-related changes in the detector signals obtained by the detector unit, such as a reduction in the signal amplitudes, within broad limits. A disadvantage of this procedure is that once the control limits of the signal processing unit have been reached, only a brief lead time remains until the position measuring device must be serviced or exchanged. In addition, the optimization of the detector signals by the signal processing unit makes it more difficult to adjust the detector unit relative to the measuring graduation inasmuch as optimal positional signals are already obtainable even after a still imprecise installation of detector signals heavily infected with errors.
German Published Patent Application No. 195 21 252 describes a position measuring device in which one or more output(s) of an output amplifier is/are able to be switched to high resistance in case of an interruption. However, this leads to an immediate emergency stop of the system so that there is no possibility for a preventive warning.